The U.S. Pat. No. of Wong, No. 5,051,275, granted Sep. 24, 1991, contains a discussion of numerous references describing the use of silicone resin as an electronic device encapsulant. Various silicone resin compounds and mixtures have been shown to have the appropriate electrical and mechanical characteristics for covering an electrical device such as an integrated circuit and protecting it from outside contaminants, while constituting a good dielectric which does not interfere with the electrical performance of the device.
A method for mounting integrated circuit chips which is becoming more widely used is known as flip-chip surface mounting. The circuitry of an integrated circuit chip and the active devices of it are typically formed on only one surface of the chip. With flip-chip surface mounting, solder bumps are used to bond the active surface of the chip to the surface of a substrate such that the active surface of the chip faces the substrate surface and is separated from it by a small distance. Such chips may be protected from outside contaminants by a hermetic package, which of course is relatively expensive, bulky, and further complicates the assembly process. It has been difficult to use silicone resin encapsulants for many of such devices because many resins, even in their uncured condition, cannot dependably fill the small gap between the semiconductor chip and the substrate surface. A larger problem is that silicone resins normally have different thermal expansion characteristics from either semiconductors or the ceramics that are typically used as substrates. Consequently, under extreme temperature conditions, the differential thermal expansion of the silicone encapsulant may break the solder bonds joining the chip to the substrate. Finally, any material used as a non-hermetic encapsulant must often withstand relatively high voltages per unit of distance, due to the increasingly close spacing of conductors on integrated circuits, as well as the absolute voltages developed.
There is therefore a continued long-felt need for dependable encapsulations for electronic devices, particularly encapsulations for flip-chip surface mounted integrated circuits that are subjected to relatively high electrical voltages and to relatively wide changes of temperature.